How to Use a Refractometer for Salinity

(Image: Aquarium image by crossgolfing from

Salinity is one of the most important parameters measured in aquariums and saltwater exhibits. Depending on the location, annual rainfall and temperature, natural sea water typically measures a salinity of 32 to 37 parts per thousand (ppt), or at a "specific gravity" of approximately 1.024 to 1.027. Frequent measuring of closed water systems with a refractometer can help aquarists keep track and maintain the salinity at which their life forms will thrive. Salinity can be reduced by adding reverse osmosis (R.O.) or distilled water, or raised by adding natural or synthetic saltwater.

Things You'll Need

  • Refractometer
  • Eyedropper
  • R.O. or distilled water
  • Natural or synthetic saltwater

Take a small sample of water from your aquarium or exhibit using an eyedropper or small syringe (no needle).

Hold the refractometer level, open the lid to the meter, and drop a few drops of water onto the meter. Then, gently close the lid.

Looking through the eyepiece, turn the focus knob until the numbers appear clearly. The upper field of the meter is blue, and the lower is white. The line where the blue turns white measures your specific gravity or salinity, depending on the model of your refractometer.

Adjust your aquarium water accordingly. If your salinity reads too high, add small amounts of R.O. or distilled water. If it measures too low, perform a 20 percent water change with saltwater of the correct salinity.

Remeasure frequently, adjusting water as needed over the next several days until optimal salinity is reached.

Tips & Warnings

  • Calibrate your refractometer periodically. This is accomplished by placing a few drops of R.O. or distilled water on the meter, then turning the calibration knob until the scale reads 0.
  • When saltwater evaporates, salt is left behind. Top off evaporated water with R.O. or distilled rather than saltwater to keep salinity in check.
  • Never allow your salinity to fluctuate more than a point or two at a time. Drastic changes in salinity will stress or may even kill living organisms.

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